The safety requirement for the risk resulting from the presence of flammable gases in low and medium strength enclosures, as industrial and civil buildings or atmospheric equipment, are very restrictive: the maximum acceptable internal pressure produced by an explosion cannot reach values greater than few tenths of bar (generally 0.1 - 0.3 bar g) for the structures involved to resist. Conversely, the overpressure developed by the explosion of a flammable mixture of fuel and air homogeneously filling the entire enclosure is of several bar. In many real cases, a small amount of fuel is released but it may form stratified (to floor) or layered (to ceiling) clouds, with portions of fuel within flammable limits. The overpressure from the explosion of these small amounts of fuel can still be higher then the values that can be tolerated by the structures involved. This is often the case also because the safety guidelines impose the adoption of alarms and interlock systems to reduce the total amount of fuel available in the explosion. The aim of this work is to extent the vent sizing correlations developed for homogenous fuel air mixture completely filling the system of containment to stratified or layered mixtures by modifying the explosion index KG to keep into account for the presence of partial filling of volume by the fuel mixture.